This invention relates to molten aluminum, and more particularly, it relates to an improved crucible for use with molten metals such as molten aluminum to provide for extended delivery time.
As noted in U.S. Pat. No. 6,049,067, incorporated herein by reference, aluminum is frequently delivered to customers in molten form. The benefits are substantial energy savings and product availability in a ready-for-use (molten) condition. Trailer mounted transport crucibles are used for this purpose. Since the heat loss from these crucibles is high, transport time is limited to a few hours, and considerable superheat must be added to the metal to ensure delivery at minimum acceptable temperature. It is common practice to heat molten aluminum to temperatures above 1700xc2x0 F. for the purpose of adding sufficient superheat. Direct impingement gas fired burners are used for this purpose.
Further, as noted, high temperature is undesirable because the resulting increase in metal oxidation rate generates skim. Melt loss can exceed 10%. Further, metal quality rapidly deteriorates since hydrogen solubility in aluminum is an exponential function of temperature, and oxides are formed. Refractory life is reduced by high temperature, and wall accretions build up and limit crucible metal capacity. The hazards associated with handling molten aluminum increase significantly with elevated temperature.
Another problem with molten metal such as molten aluminum involves transferring molten metal to and from the container or crucible because this requires the control of metal flow rate. The flow rate control is needed for operating, quality and safety considerations. The conventional means for controlling metal flow rate, for example, from a ladle by gravity includes varying the area available for metal flow. That is, when an orifice is positioned in the bottom of a ladle the size of the area of the orifice is changed to change the molten metal flow rate. Conventional means used to change the orifice area include a tapered rod or sometimes a slide gate. However, these provide no means for molten metal flow rate other than by varying the orifice area. Thus, when the ladle is full of molten metal, there is great force on the orifice, which when opened results in metal splashing and a hazardous situation. Further, there is an increase in oxides and reduced metal quality, particularly with molten aluminum, which readily oxidizes.
There is a need for a molten metal dispensing system which provides greater flow control and minimizes splashing. Further, there is a need for a heat sink to maintain molten metal at a temperature during transportation thereof.
It is an object of the invention to provide an improved container for molten metal.
It is another object of the invention to provide a container capable of extending delivery time for molten metal.
It is still another object of the invention to use pockets of metal or metal alloys having suitable heats of solidification to add heat to the molten metal during delivery.
These and other objects will become apparent from the specification, drawings and claims appended hereto.
Another embodiment of the invention contemplates a method of heating a body of molten metal in a crucible to add heat using heat of solidification to offset losses encountered during transportation or in holding in the crucible. The method comprises providing a crucible containing a body of molten metal, the crucible having a bottom and sides joined together to contain said molten metal, the sides having a liner comprised of a refractory substantially inert to the molten metal. The liner contains at least one pocket of a metal or metal alloy having a melting point above that of the molten metal contained in the crucible. Heating means such as electric heaters are provided in the pocket for heating the metal or metal alloy to its melting point. As the metal or metal alloy solidifies, it supplies heat to the body of molten metal. That is, the metal or metal alloy gives up heat of transformation to the molten metal in the crucible thereby maintaining the molten metal at temperature for a greater period of time.
The invention also includes an improved container for containing molten metal which may be employed to maintain the molten metal at target temperature longer.